 The computer system’s I/O architecture is its interface to the outside world.
 This architecture provides a systematic means of controlling interaction with the outside world
and provides the operating system with the information it needs to manage I/O activity effectively.
 There are three principle I/O techniques:
1.
programmed I/O
2.
interrupt-driven I/O
3.
direct memory access.
Input/Output Problems:
1.
Wide variety of peripherals
- Delivering different amounts of data
- At different speeds
- In different formats
2.
All slower than CPU and RAM
3.
Need I/O modules
Input/Output Module:
 Interface to CPU and Memory.
Interface to the processor and memory via the system bus or central switc h

Interface to one or more peripherals
Interface to one or more peripheral devices by tailored data links
7.1 External devices
 Human readable: Suitable for
communicating with the computer user
(Screen, printer, keyboard).
 Machine readable: Suitable for
communicating with equipment
(Monitoring and control)
 Communication: Suitable for
communicating with remote devices
(Modem, Network Interface Card (NIC).

Fig.(7.2) show Block Diagram of an External Device
The interface to the I/O module is in the form of:
 Control signals:determine the function that the device will perform, such as send data to the
I/O module (INPUT or READ),
accept data from the I/O module (OUTPUT or WRITE),
report stat.
 Data: are in the form of a set of bits to be sent to or received from the I/O module.
 Status signals: indicate the state of the device, to show whether the device is ready for data
transfer.
 Examples of interface between the I/O module and the external device are:
- Keyboard/Monitor: The most common means of computer/user interaction is a
keyboard/monitor arrangement. The user provides input through the keyboard. This input is
then transmitted to the computer and may also be displayed on the monitor.
- Disk Drive: A disk drive contains electronics for exchanging data, control, and status signals
with an I/O module plus the electronics for controlling the disk read/write mechanism.
7.2 I/O MODULES
 I/O module function:
- Control and timing
- Processor communication
- Device communication
- Data buffering
- Error detection
 I/O module must communicate with the processor and with the external device.
Processor communication involves the following:
- Command decoding: The I/O module accepts commands from the processor
- Data: Data are exchanged between the processor and the I/O module over the data bus.
- Status reporting: Because peripherals are so slow, it is important to know the status of the I/O module
- Address recognition: Just as each word of memory has an address, so does each I/O device.
 The control of the transfer of data from an external device to the processor
might involve the following sequence of steps:
1.The processor interrogates the I/O module to check the status of the attached device.
2. The I/O module returns the device status.
3. If the device is operational and ready to transmit, the processor requests the transfer of data, by means of a command
to the I/O module.
4. The I/O module obtains a unit of data (e.g., 8 or 16 bits) from the external device.
5. The data are transferred from the I/O module to the processor.
I/O Module Structure

Figure 7.3 provides a general block
diagram of an I/O module.
The module connects to the rest of
the computer through a set of
signal lines (e.g., system bus lines).
 Data transferred to and from
the module are buffered in one
or more data registers.
 A status register may also
function as a control register,
to accept detailed control
information from the processor.
 The logic within the module
interacts with the processor via
a set of control lines .
 Each I/O module has a unique
address or, if it controls more
than one external device
Input output techniques
 There are three way of
input output techniques:
1.
Programmed
2.
Interrupt driven
3.
Direct Memory Access
(DMA)
Programmed I/O

CPU has direct control over I/O
- Sensing status
- Read/write commands
- Transferring data
•
CPU waits for I/O module to complete operation
Programmed I/O –details
•
•
•
•
•
•
•
CPU requests I/O operation
I/O module performs operation
I/O module sets status bits
CPU checks status bits periodically
I/O module does not inform CPU directly
I/O module does not interrupt CPU
CPU may wait or come back later
I/O command
Addressing I/O Devices
•
CPU issues address
- Identifies module (& device if >1 per module)
•
CPU issues command
- Control - telling module what to do -e.g. spin up disk
- Test - check status - e.g. power? Error?
- Read/Write
- Module transfers data via buffer from/to device



Under programmed I/O data transfer is very like
memory access (CPU viewpoint)
Each device given unique identifier
CPU commands contain identifier (address)
I/O Mapping
 Memory mapped I/O
- Devices and memory share an address space
- I/O looks just like memory read/write
- No special commands for I/O
- Large selection of memory access commands available

Isolated I/O
- Separate address spaces
- Need I/O or memory select lines
- Special commands for I/O
- Limited set
Interrupt Driven I/O



Overcomes CPU waiting
No repeated CPU checking of device
I/O module interrupts when ready
Interrupt Driven I/O basic Operation





CPU issues read command
I/O module gets data from peripheral whilst CPU
does other work
I/O module interrupts CPU
CPU requests data
I/O module transfers data
CPU view point




Issue read command
Do other work
Check for interrupt at end of each instruction cycle
If interrupted:- Save context (registers)
- Process interrupt
- Fetch data & store
 See Operating Systems notes
Identifying Interrupting Module









Different line for each module
- PC
- Limits number of devices
Software poll
- CPU asks each module in turn
- Slow
Daisy Chain or(Hardware poll ,vectored)
—Interrupt Acknowledge sent down a chain
—Module responsible places vector on bus
—CPU uses vector to identify handler routine
• Bus Master (bus arbitration). ( vectored )
—Module must claim the bus before it can raise interrupt
—e.g. PCI & SCSI
When an I/O device completes an I/O operation, the following sequence of hardware events occurs:
1.The device issues an interrupt signal to the processor.
2. The processor finishes execution of the current instruction before responding to the interrupt
3. The processor tests for an interrupt, determines that there is one, and sends an acknowledgment signal to
the device that issued the interrupt.
4. The acknowledgment allows the device to remove its interrupt signal. The processor now needs to prepare
to transfer control to the interrupt routine. To begin, it needs to save information needed to resume the
current program at the point of interrupt.
(The minimum information required is (a) the status of the processor, which is contained in a register called the program status word (PSW),
and (b) the location of the next instruction to be executed, which is contained in the program counter. These can be pushed onto the system control stack)
5. The processor now loads the program counter with the entry location of the interrupt-handling program
that will respond to this interrupt. Depending on the computer architecture and operating system design
6. At this point, the program counter and PSW relating to the interrupted program have been saved on the
system stack.
7. The interrupt handler next processes the interrupt. This includes an examination of status information
relating to the I/O operation or other event that caused an interrupt.
8. When interrupt processing is complete, the saved register values are retrieved from the stack and restored
to the registers.
9. The final act is to restore the PSW and program counter values from the stack
7.5 DIRECT MEMORY ACCESS


Interrupt driven and programmed I/O
require active CPU intervention
- Transfer rate is limited
- CPU is tied up
 DMA Function
- Additional Module (hardware) on bus
- DMA controller takes over from CPU
for I/O
DMA Operation

CPU tells DMA controller:- Read/Write
- Device address
- Starting address of memory block for data
- Amount of data to be transferred

CPU carries on with other work

DMA controller deals with transfer

DMA controller sends interrupt when finished
DMA Transfer Cycle Stealing
 DMA controller takes over bus for a cycle( cycle stealing )
 Transfer of one word of data
 Not an interrupt
- CPU does not switch context
 CPU suspended just before it accesses bus
- i.e. before an operand or data fetch or a data write
 Slows down CPU but not as much as CPU doing transfer
Intel 8237A DMA Controller



Interfaces to 80x86 family and DRAM
When DMA module needs buses it sends HOLD signal to processor
CPU responds HLDA (hold acknowledge)
- DMA module can use buses

E.g. transfer data from memory to disk
1. Device requests service of DMA by pulling DREQ (DMA request) high
2. DMA puts high on HRQ (hold request),
3.CPU finishes present bus cycle (not necessarily present instruction) and puts high on HDLA (hold
acknowledge). HOLD remains active for duration of DMA
4. DMA activates DACK (DMA acknowledge), telling device to start transfer.
5. DMA starts transfer by putting address of first byte on address bus and activating MEMR; it then activates
IOW to write to peripheral. DMA decrements counter and increments
address pointer. Repeat until count reaches zero.
6. DMA deactivates HRQ, giving bus back to CPU.
Fly-By



While DMA using buses processor idle
Processor using bus, DMA idle Known as fly-by DMA controller
Data does not pass through and is not stored in DMA chip
- DMA only between I/O port and memory
- Not between two I/O ports or two memory locations
 Can do memory to memory via register
 8237 contains four DMA channels
- Programmed independently
- Any one active
- Numbered 0, 1, 2, and 3
Five control/command registers

The 8237 has a set of five control/command registers to program and control DMA operation over one of its
channels (Table 7.2):
1. Command: The processor loads this register to control the operation of the DMA.D0 enables a memory-tomemory transfer, in which channel 0 is used to transfer a byte into an 8237 temporary register and channel 1 is used
to transfer the byte from the register to memory. When memory-to-memory is enabled, D1 can be used to disable
increment/decrement on channel 0 so that a fixed value can be written into a block of memory. D2 enables or
disables DMA.
2. Status: The processor reads this register to determine DMA status.
- Bits D0–D3 are used to indicate if channels 0–3 have reached their TC (terminal count).
- Bits D4–D7 are used by the processor to determine if any channel has a DMA request pending.
3. Mode: The processor sets this register to determine the mode of operation of the DMA. Bits D0 and D1 are used
to select a channel. The other bits select various operation modes for the selected channel.
Bits D2 and D3 determine if the transfer is a from an I/O device to memory (write) or from memory to I/O (read), or
a verify operation. If D4 is set, then the memory address register and the count register are reloaded with their
original values at the end of a DMA data transfer. Bits D6 and D7 determine the way in which the 8237 is used. In
single mode, a single byte of data is transferred. Block and demand modes are used for a block transfer, with the
demand mode allowing for premature ending of the transfer.
4. Single Mask: The processor sets this register. Bits D0 and D1 select the channel. Bit D2 clears or sets the mask
bit for that channel. It is through this register that the DREQ input of a specific channel can be masked
(disabled) or unmasked (enabled).While the command register can be used to disable the whole DMA chip, the
single mask register allows the programmer to disable or enable a specific channel.
5. All Mask: This register is similar to the single mask register except that all four channels can be masked or
unmasked with one write operation.